This invention relates to the field of electronic distance measuring systems having utility in the aircraft guidance and landing art.
Active distance measuring equipment (DME), in which an airborne electronic source of radiating energy is used to measure the distance of an aircraft from a selected cooperating ground station or transponder, is an accepted part of the electronic assistance to aviators art. A precise measurement of distance between a landing aircraft and the end of the aircraft's intended landing runway is an example of the measurement accomplished with such equipment. Such measurement is of vital interest to the aircraft's pilot, especially during nighttime and inclement weather landing events and information of a related nature is also needed during precision flight missions.
In the modern military environment, however, the advent of signal-seeking missiles and ground-based signal detection apparatus has made the use of active or signal-emitting distance measuring equipment undesirably hazardous in many combat situations. Even in non-combat and civilian aviation environments the ever-increasing problem of signal spectrum allocation and interference between signals has made the use of active signal emitting equipment a practice to be reduced whenever possible, particularly within the limited physical separation confines of an aircraft.
The present invention offers an alternate arrangement for distance measuring, an arrangement which is energy radiation-free and therefore passive rather than active in nature with respect to the aircraft whose distance is being measured. Moreover, the disclosed arrangement may employ equipment already provided for aircraft landing assistance as a source of ground-based illumination for the distance measuring function.
The herein disclosed system is based on the simple geometric principle that an object rotating at a fixed angular velocity includes a radius vector, connecting with the axis of rotation, which rotates at the fixed angular velocity. Points along this radius vector however move with a scalar velocity that is directly proportional to the point's distance from the axis of rotation, i.e., distance along the radius vector. For example, the scalar velocity of a point at the circumference of a rotating wheel is greater than the scalar velocity of a point located near the wheel's center even though both have the same angular velocity about the wheel's center. In the present invention this concept is applied to a narrow beam of radio frequency energy rotating at a fixed and known angular velocity.
The patent art shows examples of apparatus which is of general background interest with respect to the present invention. Included in this art is U.S. Pat. No. 4,438,439 issued to J. S. Shreve which concerns a self-survey method and apparatus in which a passive station may locate itself with respect to a stationary or moving transmitting station which provides a narrow scanning beam. The Shreve passive station receives direct pulses from the transmitting station and receives reflected pulses from a set of scattering reflecting objects. In the Shreve invention the location of the passive station is determined by a plurality of distance and angle measurements with respect to these signals reaching the passive station from the transmitting station. Since the Shreve invention does not contemplate a measurement of time difference of arrival between points of known separation distance and since the Shreve invention is based on propagation or travel times rather than scanning times, the present invention is readily distinguished from concepts disclosed in the Shreve patent.
Also included in this patent art of general interest is the U.S. Pat. No. 5,053,784 of L. Hippelainen which concerns an apparatus and method for measuring the azimuth and elevation of an object through the use of plural radiating elements. Since the present invention is concerned with a distance measuring arrangement, a ready distinction over the Hippelainen disclosure is apparent.